Recently, in optical communications, as a method capable of rapidly increase transmission capacities, many researches and developments of optical wavelength division multiplexing communications have been positively performed and practically used. An optical wavelength division multiplexing communication corresponds to such an optical communication that, for instance, a plurality of lights having different wavelengths from each other are multiplexed and then the multiplexed light is transferred. In such an optical wavelength division multiplexing communication system, in order to demultiplex lights with the different wavelengths each other from the transmitted multiplexed light, an optical transmission device and the like which may transmit, therethrough only such lights having a predetermined wavelengths is necessary.
As one example of such an optical transmission device, a planar lightwave circuit (PLC) as shown in FIG. 15 has been proposed. The planar lightwave circuit indicated in FIG. 15 is called as an arrayed waveguide grating (AWG). This arrayed waveguide grating includes a waveguide forming region 10 made of silica-based glass is formed on a substrate 1 made of, for example, silicon. The waveguide forming region 10 of the arrayed waveguide grating has such a waveguide structure as indicated in FIG. 15.
In other words, the waveguide structure of the arrayed waveguide grating comprises at least one optical input waveguide 2, a first slab waveguide 3, an arrayed waveguide 4, a second slab waveguide 5, and a plurality of optical output waveguides 6. The first slab waveguide 3 is connected to an output side of the at least one optical input waveguide 2. The arrayed waveguide 4 is connected to an output side of the first slab waveguide 3. The second slab waveguide 5 is connected to an output side of the arrayed waveguide 4. The plurality of optical output waveguides 6 are connected to an output side of the second slab waveguide 5, and arranged side by side.
The arrayed waveguide 4 transmits a light which is outputted from the first slab waveguide 3. This arrayed waveguide 4 is formed in such a manner that a plurality of channel waveguides 4a are arranged side by side. The lengths of adjacent channel waveguides 4a are different by a set amount (xcex94L) from each other.
The channel waveguides 4a which constitute the arrayed waveguide 4 are normally a large number, for example, 100 pieces. Also, a plurality of optical output waveguides 6 are provided in correspondence with a total number of signal lights having different wavelengths. In the drawings, for the sake of simple illustrations, a plurality of channel waveguides 4a, optical input waveguides 2, and optical output waveguides 6 are represented in a simple abbreviated manner.
For instance, while an optical fiber (not shown) on the transmission side is connected to one of the optical input waveguides 2, multiplexed light may be conducted, to this optical fiber. This multiplexed light is conducted via one of the optical input waveguides 2 to the first slab wavelength 3, and then, is entered into the arrayed waveguide 4, while this multiplexed light is widened due to the diffraction effect thereof, and thereafter, the widened light is transferred via the arrayed waveguide 4.
This multiplexed light which has been transferred via the arrayed waveguide 4 is reached to the second slab waveguide 5, and furthermore, light beams are condensed to each of the optical output waveguides 6, and then, the condensed lights are outputted therefrom. In this case, since the lengths of all of these channel waveguides 4a of this arrayed waveguide 4 are different from each other, phases of the individual light after being transferred via the arrayed waveguide 4 are shifted. Then, a wavefront of condensed light is tilted in response to this shift amount each other, and a position where the lights are condensed may be determined based upon this tilt angle.
It should also be noted that assuming now that an angle (diffraction angle) of light to be condensed is set as xe2x80x9cxcfx86xe2x80x9d when this light is inputted from the arrayed waveguide 4 to the second slab waveguide 5, formula 1 may be substantially satisfied between this diffractive angle xe2x80x9cxcfx86xe2x80x9d and a center wavelength (center wavelength of light transmission) xcex of the light to be condensed:
nsxc3x97dxc3x97sinxcfx86+ncxc3x97xcex94L=mxc3x97xcexxe2x80x83xe2x80x83(formula 1) 
where symbol xe2x80x9cnsxe2x80x9d shows equivalent index of both the first and second slab waveguide; symbol xe2x80x9cdxe2x80x9d represents an interval between edge portions of the mutual channel waveguides on the side of the first and second slab waveguide; symbol xe2x80x9cxcfx86xe2x80x9d denotes a diffraction angle; symbol xe2x80x9cncxe2x80x9d shows an equivalent index of the arrayed waveguide; symbol xe2x80x9cxcex94Lxe2x80x9d shows a difference between lengths of the adjacent channel waveguides; and also, symbol xe2x80x9cmxe2x80x9d indicates a diffraction order.
In this case, assuming now that a wavelength is set to xe2x80x9cxcex0xe2x80x9d at the diffraction angle xcfx86=0, this wavelength xe2x80x9cxcex0xe2x80x9d is substantially expressed by the below-mentioned formula (2). Generally speaking, this wavelength xe2x80x9cxcex0xe2x80x9d is referred to as a center wavelength of an arrayed waveguide grating.
xcex0=ncxc3x97xcex94L/mxe2x80x83xe2x80x83(formula 2) 
Also, as shown in FIG. 18, assuming now that a light condensed point of such an arrayed waveguide grating is set to a point xe2x80x9cOxe2x80x9d at the diffraction angle xcfx86=0, as to a condensed position of such light having another diffraction angle xe2x80x9cxcfx86pxe2x80x9d, this light is condensed at a position xe2x80x9cPxe2x80x9d which is different from the above-explained point xe2x80x9cO.xe2x80x9d This position xe2x80x9cPxe2x80x9d corresponds to such a position which is shifted from the point xe2x80x9cOxe2x80x9d along an X direction. In this case, assuming now that a distance between these points xe2x80x9cOxe2x80x9d and xe2x80x9cPxe2x80x9d along the X direction is set to xe2x80x9cxxe2x80x9d, the below-mentioned formula (3) be substantially satisfied between the distance xe2x80x9cxxe2x80x9d and the wavelength xe2x80x9cxcexxe2x80x9d.                                           ⅆ            χ                                ⅆ            λ                          =                                                                              L                  f                                ·                Δ                            ⁢                              xe2x80x83                            ⁢              L                                                      n                s                            ·                              ⅆ                                  ·                                      λ                    0                                                                                ⁢                      n            g                                                        (                      formula            ⁢                          xe2x80x83                        ⁢            3                    )                .            
where, symbol xe2x80x9cLfxe2x80x9d indicates a focal distance of the second slab waveguide 5; and symbol xe2x80x9cngxe2x80x9d represents a group index of the arrayed waveguide 4. It should also be noted that the group index xe2x80x9cngxe2x80x9d of this arrayed waveguide 4 may be given by the equivalent index xe2x80x9cncxe2x80x9d of the arrayed waveguide 4 in accordance with the following formula (4).                               n          g                ⁢                  xe2x80x83                =                  xe2x80x83                ⁢                              n            c                    ⁢                      xe2x80x83                    -                      xe2x80x83                    ⁢                                    λ              0                        ⁢                          xe2x80x83                        ⁢                                          ⅆ                                  n                  c                                                            ⅆ                λ                                                                                  (                      formula            ⁢                          xe2x80x83                        ⁢            4                    )                .            
The above-explained formula (3) implies such a fact that since the input end of the optical output waveguide 6 is arranged/formed at such a position which is separated from the focal point xe2x80x9cOxe2x80x9d of the second slab waveguide 5 by a distance xe2x80x9cdxxe2x80x9d along the X direction, such light having a wavelength different by xe2x80x9cdxcexxe2x80x9d can be derived.
Also, the relationship established in the above-explained formula (3) may be similarly established as to the first slab waveguide 3. That is to say, for example, assuming now that a focal center of the first slab waveguide 3 is set to a point xe2x80x9cOxe2x80x9d and also, such a point is set to another point xe2x80x9cPxe2x80x2xe2x80x9d whose location is shifted by a distance xe2x80x9cdxxe2x80x2xe2x80x9d from this point xe2x80x9cOxe2x80x2xe2x80x9d along the X direction, when light is entered into this point xe2x80x9cPxe2x80x2xe2x80x9d, a wavelength of an output may be shifted by xe2x80x9cdxcexxe2x80x2.xe2x80x9d When this relationship is expressed by a formula, the below-mentioned formula (5) may be substantially obtained:                                           ⅆ                          χ              xe2x80x2                                            ⅆ                          λ              xe2x80x2                                      ⁢                  xe2x80x83                =                  xe2x80x83                ⁢                                                                              L                  f                  xe2x80x2                                ·                Δ                            ⁢                              xe2x80x83                            ⁢              L                                                      n                s                            ·                              xe2x80x83                            ⁢                              ⅆ                                  ·                                      λ                    0                                                                                ⁢                      xe2x80x83                    ⁢                      n            g                                                        (                      formula            ⁢                          xe2x80x83                        ⁢            5                    )                .            
where, symbol xe2x80x9cLfxe2x80x2xe2x80x9d indicates a focal distance of the first slab waveguide 33.
The above-explained formula (5) implies such a fact that since the output end of the optical input waveguide 2 is arranged/formed at such a position which is separated from the focal point xe2x80x9cOxe2x80x2xe2x80x9d of the first slab waveguide 3 by a distance xe2x80x9cdxxe2x80x2xe2x80x9d along the X direction, such light having a wavelength different by xe2x80x9cdxcexxe2x80x2xe2x80x9d can be derived in the optical output waveguide 6 which is formed at the above-explained focal point xe2x80x9cOxe2x80x9d.
In FIG. 15, a demultiplexing function of an arrayed waveguide grating is schematically illustrated. In the case that a multiplexed light having a plurality of wavelengths xcex1, xcex2, - - - , xcexn (symbol xe2x80x9cnxe2x80x9d is an integer larger than, or equal to 2) is entered into one of optical input waveguides 2, and the demultiplexed lights are outputted from the optical output waveguides 6 which are different from each other every wavelengths.
Also, since the arrayed waveguide grating utilizes the principle idea of the reciprocity characteristic (reversibility) of the optical circuit, this arrayed waveguide grating may own both a function as a multiplexer, and also another function as a demultiplexer. As a consequence, contrary to FIG. 15, in such a case that a plurality of lights having wavelengths different from each other are entered from the respective optical output waveguides 6 into this arrayed waveguide grating, the plurality of lights are traveled through such a transfer path opposite to the above-described transfer path, and then, are multiplexed with each other by way of both the arrayed waveguide 4 and the first slab waveguide 3. Then, the multiplexed light is outputted from one of the optical input waveguides 2.
According to an aspect of the present invention, an arrayed waveguide grating optical multiplexer/demultiplexer comprises:
a planar lightwave circuit in which a waveguide forming region is formed on a substrate; wherein:
the waveguide forming region is comprised of:
at least one optical input waveguide;
a first slab waveguide connected to an output side of the at least one optical input waveguide;
an arrayed waveguide connected to an output side of the first slab waveguide, and includes a plurality of channel waveguides arranged side by side and has different lengths of a set amount;
a second slab waveguide connected to an output side of the arrayed waveguide; and
a plurality of optical output waveguides connected to an output side of the second slab waveguide, and arranged side by side; and
at least one of the first slab waveguide and the second slab waveguide is separated at a plane intersected to a path of light passing through the slab waveguide so as to form separated slab waveguides; and wherein:
the arrayed waveguide grating optical multiplexer/demultiplexer is further comprised of:
a slide moving member for slide-moving at least one of the separated slab waveguides along said separating plane, depending upon a temperature; and
a positional shift suppressing member for suppressing an optical axis shift along a Z direction perpendicular to substrate of the separated slab waveguides by depressing the waveguide forming region; and wherein:
a position depressing the waveguide forming region by the positional shift suppressing member is formed at such a position escaped from the optical axis of the separated slab waveguides.